1. Field of the Invention
This invention pertains to mounting boards for integrated circuit devices and for interconnection thereof. More particularly, it pertains to a logic card for interconnection of high frequency integrated circuit devices with high packing densities on the card and in which transmission line termination devices are defined in the card.
2. Review of the Prior Art
Integrated circuits (ICs) are being developed for operation at faster and faster cycle rates. Typical IC operating rates are now in the range of nanoseconds; one nanosecond in one billionth (10.sup.-9) second. IC operating rates in the range of picoseconds, i.e., trillionths (10.sup.-12) of a second, can be expected in the near future.
In the context of this invention, it will be understood that ICs operate as a complex series of switches which switch on or off depending upon whether a signal, or a combination of signals, is or is not present at the inputs to the switches. The signals are digital in that it is the presence or absence of a voltage which is important, not the magnitude of the voltage. Thus, ICs operate on, or in response to signals which shift very rapidly between "signal" and "no signal" states.
The switches and other components of ICs operate at very fast speeds. In a good conductor which does not create delays in signal propagation, electrical impulses travel at speeds approaching the speed of light; IC circuit components operate at comparable speeds. A principal practical limitation on the speed of an IC is how far a signal has to travel, within the IC, at the speed of light. The astounding speed of ICs is due to their very compact nature, such that the distances signals must travel between circuit components is very very small. The stimulus to keep inter-component distances very small has led to the development of medium scale integrated circuits (MSIs) and large scale integrated circuits (LSIs); some examples of the latter have an area measured in square inches, whereas ICs have effective areas measured in small fractions of a square inch.
Regardless of its size, any IC is a device which performs only the functions or logical operations it is specifically designed to do, either as a result of the internal arrangement of the device or as a result of the manner in which connections are made to the device. ICs, regardless of size, involve very precise, complex and costly fabrication procedures. ICs, therefore, are economically feasible, in most instances, only where they can be produced in quantity. There are many instances where the high speed operation of ICs is desired, but the number of total devices needed is too low to justify the cost of MSIs or LSIs. As a result, ICs, as distinguished from MSIs and LSIs, are available in a number of configurations (functions) for use as building blocks in the construction of larger logic circuits of limited quantity and specialized nature. Such building blocks are mounted on special circuit boards, commonly called "logic cards", and are interconnected in appropriate manners on the boards to define the overall logic circuits. The individual ICs are mounted on the boards as close together as possible to reduce, to the greatest extent possible, the lengths of the interconnections between the ICs, all so that the interconnection distances do not impose speed limitations on the overall logic circuit.
Heretofore, the ICs used in building block fashion on logic cards did not have operating rates which significantly exceeded the transmission time between individual ICs on a logic card, and the relatively low density at which such ICs were packed or mounted on logic cards was not a serious problem. Recently, however, ICs having very fast operating rates have been developed; emitter coupled logic (ECL) ICs are an example of these new ICs. ECL ICs operate at speeds in the nanosecond range. (For example, Fairchild Semiconductor's F100K ECL ICs have a response time of 0.7 nanoseconds). When ECL ICs are mounted on existing logic cards, two problems are presented. The first problem is that the interconnecting paths between individual ICs is so long that the delays of signal transmission between ICs, simply as a matter of distance, are significant. The second problem is that the signal frequencies (the rate at which the signal voltage shifts from a "no-signal" state to a "signal" state and back) are so high that the interconnections between ICs begin to function as transmission lines, thus giving rise to the subsidiary problems of signal reflection and degradation and of propagation delays, all caused by frequency related impedance mismatches. These problems have been solved, after a fashion, by the mounting on the surface of the logic cards with the ICs of transmission line termination devices, such as resistors or resistor networks. The individual resistors couple the IC leads to the interconnections between ICs and serve as transmission line termination impedances which match the IC impedances to the interconnection impedances; this is done to give increased signal transmission efficiency. Increased transmission efficiency between ICs makes the signals move faster and better. The difficulty which the use of such resistors and resistor networks on logic cards is that they occupy space on the cards. Thus, the several ICs on the cards must be spaced farther apart, and the delays attributable solely to distance remain or are increased. The result is that overall circuits composed of interconnected high speed ICs have net operating rates significantly lower than the operating rates of the component ICs themselves, solely because of the distances which the signals must travel between ICs.
In view of the foregoing, a need exists for improved ways to closely mount high speed ICs on logic cards, thereby to reduce to the greatest extent possible the distances which signals must travel along interconnections between ICs, while still providing compensation for the transmission line effects which arise in such interconnections by virtue of the signal frequencies involved.